Substance flow analysis on the national scale may be carried out by calculating the national balance: subtracting all the heavy-metal flows leaving agriculture from all the heavy-metal flows entering agriculture. Thus the total net input of heavy metals to the (agricultural) soil gives an overview of the 'average burden' on 'average agricultural soil' by applying national statistics on feedstuff's, mineral fertilizers, animal manure, agricultural products (milk, meat, crops) and so on. Using averages of annual sales, compositions (for example, of crops or fertilizers), application rates and yields per crop, the mean annual loads in a certain region can be calculated. The information on composition may be obtained by direct measurements or by using statistical and bookkeeping data, or (usually) a combination of the two. In either case the numbers give a 'snapshot' at a certain point in time. Regular monitoring is the only way to check on variability and time trends.
Dutch, Danish and Finnish studies illustrate the large uncertainties in quantifying metal flows on the national scale. Inputs of heavy metals by (transboundary) air pollution and imports of feed concentrates (containing Cd, Cu, Zn) and phosphate rock (containing Cd) are striking (Poppe et al 1994; de Boo 1995). In Denmark, the effects of legislation on decreasing the Cd surplus on the balance have been quite positive. The prohibition of cadmium in pigments and other products, combined with stack gas cleaning, has greatly reduced atmospheric emissions in the last few years. Moreover, the Cd contents in phosphate fertilizers have also decreased (Hovmand 1984; Tjell and Christensen 1992). In Finland, the low deposition rate of Cd and the exceptionally low Cd input with phosphate fertilizers has (on average) led to a steady-state situation (that is, no net accu mulation) for Cd in arable soils. However, copper and zinc added in large amounts to feeds result in large Cu and Zn flows in animal manure (Makela-Kurtto 1996). Average numbers on net accumulation of heavy metals in soil are given in Table 33.1.
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